A tunable external cavity laser light source is typically constructed from a reflective cavity having a gain chip and a wavelength selective filter that suppresses light of a wavelength outside a narrow band of wavelengths. The light that is passed by the filter is amplified by the gain chip via stimulated emission. A portion of the light is removed from the cavity and becomes the output beam from the light source. For any given cavity length, the gain chip can lase in a number of different modes. Ideally, only one of these modes is selected by the wavelength filter. If the passband of the filter is too great, the light source can jump between modes of the cavity with corresponding jumps in wavelength and other properties of the output light beam. Accordingly, it is advantageous to provide a wavelength filter with as narrow a passband as possible, while providing a means to change the center frequency of the passband when the light source is tuned to a new frequency.